Cancer immunotherapy which utilizes tumor antigen-specific, depleting antibodies have been explored with great success (see, e.g., reviews by Blattman and Greenberg, Science, 305:200, 2004; Adams and Weiner, Nat Biotech, 23:1147, 2005). Tumor antigen-specific, depleting antibody therapies deplete tumor cells by, e.g., antibody-directed cellular cytotoxicity (ADCC), complement dependent cytotoxicity (CDC), induction of apoptosis in tumor cells, and recruitment of T cells responding to tumor antigens released upon antibody-mediated tumor lysis. A couple of examples of tumor antigen-specific, depleting antibodies are HERCEPTIN® (anti-HER2/neu mAb) (Baselga et al., J Clin Oncology, Vol 14:737, 1996; Baselga et al., Cancer Research, 58:2825, 1998; Vogal et al. J Clin Oncology, 20:719, 2002) and RITUXIN® (anti-CD20 mAb) (Colombat et al., Blood, 97:101, 2001).
The current tumor antigen-specific, depleting antibody therapies have clearly made a mark in oncology treatment. Unfortunately, as monotherapy the naked antibodies often lack sufficient potency to kill meaningful amounts of tumor and generally work in about 50% of patients and with partial response, as many tumors do not respond to, or relapse after such therapies. As such, there continues to be extensive research directed toward evaluating and improving the response rates associated with such therapies.
Tumor necrosis factor is a rapidly growing superfamily of cytokines (“TNFSF”) that interact with a corresponding superfamily of receptors (“TNFSFR”). Since the discovery of tumor necrosis factor-alpha (“TNF-α”) about 25 years ago, the TNFSF has grown to a large family of related proteins consisting of over 20 members that signal through over 30 receptors (see, e.g., “Therapeutic Targets of the TNF Superfamily”, edited by Iqbal S. Grewal, Landes Bioscience/Springer Science+Business Media, LLC dual imprint/Springer series: Advances in Experimental Medicine and Biology, 2009). Members of TNFSF have wide tissue distribution and TNFSF ligand-receptor interactions are involved in numerous biological processes, ranging from hematopoiesis to pleiotropic cellular responses, including activation, proliferation, differentiation, and apoptosis. TNFSF member ligand-receptor interactions have also been implicated in tumorigenesis, transplant rejection, septic shock, viral replication, bone resorption and autoimmunity. The particular response depends upon the receptor that is signaling, the cell type, and the concurrent signals received by the cell.
With the exception of lymphotoxin-α(“TNF-β”) which is produced as a secreted protein, TNFSF proteins are synthesized as type 2 membrane proteins and fold into conserved β-pleated sheet structures that trimerize. These ligands contain a relatively long extracellular domain and a short cytoplasmic region (Gruss and Dower, Blood, 85:3378-3404, 1995). Their extracellular domains can be cleaved by specific metalloproteinases to generate a soluble molecule. In general, cleaved and noncleaved ligands are active as noncovalent homotrimers, although some members can also exist as heterotrimers. Both membrane bound and secreted ligands are expressed by a variety of normal and malignant cell types (Aggarwal, B B. Nat Rev Immunol, 3:745-756, 2003). Since most of TNFSF member ligands are expressed as transmembrane cell surface proteins, it is believed they are acting at a local level.
TNFSFRs are type I membrane proteins characterized by the presence of a distinctive cysteine-rich domain in their extra-cellular portion (Aggarwal et al, Nat Rev Immunol, 3:745-756, 2003). Most TNFSF member ligands bind to one distinct receptor; however some of the TNFSF member ligands are able to bind to multiple TNF receptors (e.g., TRAIL/Apo-2L) is known to bind five receptors (DR4, DR5, DCR1, DCR2 and OPG)), explaining to some extent the apparent disparity in the numbers of TNFSF member receptors and ligands. TNFSFRs exert their cellular responses through signaling sequences in their cytoplasmic regions.
Based upon their cytoplasmic sequences and signaling properties, the TNFSFRs can be classified into three major groups (Dempsey et al., Cytokine Growth Factor Rev, 14:193-209, 2003). The first group includes receptors that contain a death domain (DD) in their cytoplasmic tail. These receptors include CD95, TNFR1, DR3, DR4, DR5 and DR6. Binding of TNFSF member ligands to their DD containing receptors causes complex signaling through adaptor proteins, such as tumor necrosis factor receptor-associated death domain (TRADD), resulting in activation of the caspase cascade and apoptosis of the cell (Kischkel et al., Immunity, 12:611-620, 2000). The second groups of receptors contain one or more TNF receptor-associated factors (TRAF) interacting motifs (TIM) in their cytoplasmic tails. This group includes TNFR2, CD40, CD30, CD27, LT-βR, OX40, 4-1BB, BAFFR, BCMA, TACI, RANK, NGFR, HVEM, GITR, TROY, EDAR, XEDAR, RELT and Fn14. Ligand binding to TIM containing TNFSFRs induces recruitment of TRAF family members and activation of cellular signaling pathways including activation of nuclear factor-κB (NF-κB), Jun N-terminal kinase (JNK), p38, extracellular signal regulated kinase (ERK) and phosphoinisitide-3 kinase (Darnay et al., J Biol Chem, 274:7724-7731, 1999). The third group of TNF receptor family members does not contain functional intracellular signaling domains or motifs. These receptors include DcR1, DcR2, DcR3 and OPG. Although this group of receptors lacks the ability to provide intracellular signaling, they can effectively act as decoys to compete for ligand binding and block the signaling through other two groups of receptors (Gibson et al., Mol Cell Biol, 20:205-212, 2000).
As stated above, one group of TNFSFRs are death receptors and have the unique ability to transmit an intracellular death signal and TNFSF ligands such as TNF-α, CD40L, CD95L (also “FasL/Apo-1L”) and TRAIL/Apo-2L are capable of inducing apoptosis in tumor cells. Because TNF-α receptors are expressed on many tumor cells, TNF-α has been exploited for its antitumor effects. Its use as an anti-cancer agent, however, is limited because of its systemic toxicity (Feinberg et al., J Clin Oncol, 6:1328-1334, 1988). Similar to the use of TNF-α, in vivo use of CD95L is also limited by its lethal hepatotoxicity resulting from massive hepatocyte apoptosis.